This project seeks to specify the neuroendocrine bases for circannual rhythms in behavior and physiology of mammals. The model species is the golden-mantled ground squirrel Spermophilus lateralis. The great majority of prior studies of seasonality has focused on rhythms that are not completely endogenous (Type I rhythms). This project is concerned with completely endogenous circannual rhythms that persist for many cycles in the absence of environmental input (Type II rhythms). The mechanisms that generate and synchronize Type II rhythms differ from those that mediate Type I rhythms. Type II rhythms are common in long-lived mammals, including primates; perhaps the most accessible route to gaining insight into the neuroendocrine bases of Type II rhythms in human behavior and physiology is to study relevant animal model systems. Specific aims of the present project include: 1) characterization of the role of the suprachiasmatic nucleus in gating circannual rhythms of body mass, reproduction and hibernation to specific phases of each circannual cycle and determining whether a single circannual clock generates the several rhythms, 2) assessment of the effects of prolonged hypothermia on circannual rhythms: are the clock(s) that gate the body mass and reproduction rhythms arrested in squirrels that hibernate continuously for two or more years? 3) determination of the role of the suprachiasmatic nucleus in sex differences in circannual organization, 4) establishing whether changes in tissue temperature are necessary for the striking circannual modulation of circadian rhythms in locomotor activity, and 5) completion of an ongoing analysis of the neural sites at which melatonin phase shifts circannual rhythms. This research may provide insight into the proximate neural bases for human rhythms and aid in the development of hormone and light therapies for treatment of seasonal affective disorders and other seasonal disturbances.